Just checking, probably unrelated: Any other additions like custom web pages or third party plugins?
You have quite a few instances of failing query fetches:
*** [ DIAGNOSING ]: contents of /var/log/lighttpd
-rw-r--r-- 1 www-data www-data 4123 Nov 20 13:51 /var/log/lighttpd/error.log
-----tail of error.log------
(gw_backend.c.2154) response already sent out, but backend returned error on socket: unix:/var/run/lighttpd/php.socket-0 for /admin/api_db.php?getAllQueries&from=1605342564.802&until=1605860964.802&types=1,2,3,4,5,6,7,8,9,10,11,12,13&_=1605860964544, terminating connection
Those could be related to unknown status values:
*** [ DIAGNOSING ]: contents of /var/log
-rw-r--r-- 1 pihole pihole 407995 Nov 20 13:49 /var/log/pihole-FTL.log
-----tail of pihole-FTL.log------
[2020-11-20 13:44:42.421 12938M] Error: Found unknown status 12 in long term database!
[2020-11-20 13:44:42.421 12938M] Timestamp: 1605858970
[2020-11-20 13:44:42.421 12938M] Continuing anyway...
Could well be prompted by status 12, which was introduced only recently.
To preclude you accidentally spoiled your database:
Did you perhaps experiment with the corresponding ftl fix/retries_master at some time in the past?